Nonlinear response to probe vitrification


Project Heuer (Münster)

Nonlinear forced motion of particles in a glass-forming system


The complexity of the dynamics of supercooled liquids is largely related to the highly cooperative dynamics which is required to achieve structural relaxation. It is planned to study the non-linear response of probe particles via molecular dynamics simulations to unravel some key aspects of this cooperativity. As a model system we take the well-studied binary Lennard-Jones system. Beyond the determination of standard observables such as the van Hove correlation function the results shall be expressed in terms of the framework of the potential energy landscape as well as of continuous time random walk characteristics. Both approaches have proven to be very helpful to gain a better understanding of the equilibrium case. Starting from this well-founded basis we study the different regimes of forced motion, starting from linear response via weakly non-linear effects to very strong forces with individual characteristics for each regime. A closer understanding is further attempted by studying the structure and dynamics of the neighborhood of the forced probe particles. In a next step this project will be extended, on the one hand, by studying the effective attractive interaction of probe particles and, on the other hand, by analyzing the effect of oscillating forces. Finally, based on these ingredients we will perform a detailed comparison of our Lennard-Jones results with corresponding experiments by the Roling group on mixtures of ionic liquids and molecules.

P4 Heuer, Universität Münster

Nonlinear response from the perspective of energy landscapes

We study the micro- and macro-rheological response of glass-forming systems via computer
simulations. So far we have shown that even in the nonlinear response regime the response in
the micro-rheological setup can be very well interpreted and understood in terms of a continuous
time random walk (CTRW). Its temperature dependence can be related to the underlying
potential energy landscape (PEL). An explicit relation to the occurrence of dynamic heterogeneities
in the quiescent state has been formulated. Key questions for the next funding period
deal, first, with the response to external fields in charged systems. In particular the effect of
electric fields on ionic liquids will be studied. Second, we analyse the macro-rheological response
in terms of a CTRW and the underlying PEL. From studying the finite-size effects new
information about the mechanisms of the linear and nonlinear response may be gained, extending
previous work on quiescent systems along this line.